We have been involved in an ongoing effort to develop a multilocus genetic map of the mouse. Most of this work has been accomplished by the analysis of the progeny of two genetic crosses, an interspecies backcross and an intersubspecies backcross. DNAs from these mice have been typed for a variety of reference loci to permit mapping of newly defined genes of unknown map location to specific positions on the linkage map. These studies have resulted in the genetic mapping of a large number of genes including, most recently, various genes expressed in nervous tissue, transcriptional regulators, protein kinases and factors involved in bone formation. Specific map locations can be useful information since proximity to a known developmental mutation can identify such a gene as a potential candidate for the abnormal phenotype. Thus, one gene encoding a major stuctural component of cartilage was mapped near the cmd (cartilage matrix deficiency) mutation, and cmd mice were shown to have a 7 bp deletion of this gene. In another example, the murine merosin gene was mapped near dy (dystrophia muscularis) and we identified a deficiency of merosin in mutant mice suggesting that a defect in this laminin gene is responsible for this disorder. Finally, the murine interleukin 2 gamma receptor chain gene (Il2rg) was mapped to a position on the X chromosome near the xid immunodeficiency mutation, although further investigation indicated that Il2rg was not responsible for xid.